ELECTRICAL MEASUREMENTS 187 



per salt. Copper atoms exist in an equilibrium, Cu ^ Cu^"^ + 2e~. 

 Copper atoms will be deposited on the wire and removed from the wire 

 at random. Once the equilibrium is established there will be no net in- 

 crease or decrease in the amount of copper metal or Cu"^"^. Any increase 

 in the concentration of Cu++ or e~ leads to a deposition of Cu metal 

 on the wire. Now, consider a similar system containing another metal, 

 say zinc, in a solution of its salt. The same kind of equilibrium will be 

 estabhshed, Zn^Zn + + + 2e~, but the proportion of metal and ions 

 is different. If these two solutions are placed in the same container and 

 the two wires are connected outside the solution, interesting things start 

 happening. Zinc atoms are more likely than copper to exist as ions. If 

 we compare the two equations above, one is more likely to run forward, 

 the other backward. At the zinc wire, Zn + + ions depart into the solution 

 leaving behind two electrons. At the copper wire, Cu++ ions pick up 

 electrons from the wire and become Cu metal. As long as the wires are 

 connected, electrons flow through the external circuit from one electrode 

 to the other. The pair of chemical reactions, then, has generated a voltage 

 which produces a current in the external wire. Almost any metal would 

 work as one member of such a pair. In fact it need not even be a metal 

 because hydrogen gas and hydrogen ions reach the same kind of equilib- 

 rium. The hydrogen electrode is used as a reference point in potentio- 

 metric measurements. A number of kinds of biological reactions produce 

 similar effects, directly or indirectly. These reactions can be followed by 

 measuring the potentials developed. In practice one of the electrodes is 

 commonly the calomel electrode (Hg2 CI2 * KCl), and the other is a 

 platinum wire which can absorb electrons from an oxidation-reduction 

 reaction occurring in the solution. 



The potential developed in such a system is measured with a poten- 

 tiometric circuit. The voltage to be measured is compared to some 

 standard voltage by means of a bridge-like arrangement of resistors. 



The electrical measurement of pH depends on a similar electrode 

 reaction. A solution of known hydrogen ion concentration is contained 

 in a tube of glass (the glass electrode) in contact with a silver-silver 

 chloride couple. A calomel electrode is used as the other "half-cell." If 

 the glass electrode is placed in a solution containing hydrogen ions, as 

 all aqueous solutions do, hydrogen ions move through the glass mem- 

 brane. They move inward or outward, depending on the pH of the 

 solution; the result is a potential which can be measured. Within the 

 useful range, the potential developed is a remarkably consistent function 

 of the pH of the solution. 



